The influence of meridional transport on estimation of regional scale CO2 fluxes in the temperate zone of the Northern Hemisphere.
Joseph
A
Berry, Carnegie Institution, joeberry@stanford.edu
(Presenting)
Adam
Wolf, Carnegie Institution, adamwolf@stanford.edu
A theoretical treatment by Betts et al., (2004) predicts that, (and empirical analyses of observational data by Bakwin et al., (2004) and Helliker et al., (2004) demonstrate that), exchange of CO2 between the free troposphere (FT) and the atmospheric boundary layer (ABL) is on average, approximately in steady-state balance with CO2 fluxes from the surface. For example, an analysis vertical advection (derived from analysis of the water budget of the ABL) and CO2 concentration measurements over five years from the WLEF tall tower in Wisconsin match the seasonal cycle of net CO2 exchange, measured by eddy correlation, of ecosystems in the region and capture interannual variation in the seasonal pattern of CO2 exchange from year to year. To follow-up on this empirical work, we have used a global atmospheric transport model with forward simulations of atmospheric CO2 concentration and other tracers (Kawa et al., 2004) to conduct a more detailed analysis of the influence of global atmospheric transport on these regional-scale correlations. The simulations show subtle mismatches between the local fluxes and CO2 concentration gradients that appear to indicate meridional transport of CO2 concentration anomalies that lead or lag the local fluxes. We have identified that these anomalies are associated with transport of air from the ABL over continental areas of the N. Hemisphere (>30N) into the polar region (>75N) and back. This flow drives the large seasonal changes in CO2 concentration observed at arctic sampling stations - as there is little or no exchange of CO2 in this zone. Studies of preferential transport of mid-latitude atmospheric pollution to the arctic (Stohl, 2006) also lend support to this analysis. The air flowing into the polar cell mixes over 10 -20 days before joining a return flow. Therefore, the CO2 concentration of air coming back from the pole can differ from that flowing in at any given time. The total change in storage of CO2 in the arctic atmosphere is similar to the putative carbon sinks in northern forests. Thus, regional scale atmospheric balance studies will need to take meridional transport of CO2 associated with synoptic weather systems into account. This will require input from a global modeling framework.